Ultrasonic transducers, and in particular phased array ultrasonic transducers, are frequently utilized for a variety of medical and other scanning applications. For such applications, and in particular medical applications, the transducer of the ultrasonic scanner is generally positioned adjacent to a selected outer portion of the body, for example, adjacent to the chest wall to scan the heart. However, superior images can be obtained in at least some applications by positioning the transducer at the end of an endoscope which is suitably positioned in the patient's esophagus. Where this transducer or probe, which is referred to as a transesophageal probe, is utilized for scanning the heart, the procedure is referred to as transesophageal echocardiography (TEE). In order to permit multiple cross-sectional planes of the heart to be scanned, it would be desirable if the transducer array could be rotated.
However, since a probe of this type is inside the body, the probe must be sealed to protect it against attack from body fluids and acids, as well as against sterilant solutions and cleaning solutions either inside or outside the body. This requires the transducer, and any focusing lens thereon, to be enclosed and sealed within a protective housing which does not move as the transducer and lens are rotated. The housing, which is preferably of metal, may be sealed and electrically isolated from the patient by having an epoxy covering molded over it. However, neither the metal housing nor the epoxy covering will transmit ultrasonic signals from the transducer or the echo signal returned theretos. Therefore, it is necessary that an opening be formed in the housing and epoxy covering over the transducer array through which ultrasonic signals may pass.
However, this opening must also be sealed. In order to avoid acoustic distortion from the sealing medium, this seal would typically be a thin plastic film, such as for example a Mylar film, attached to the housing. Such a sealing medium, which is generally flexible, also protects the body from irritation as a result of probe rotation. However, since air has different acoustic properties than the focusing lens and the body being scanned, and thus causes undesired reflection of acoustic waves passed therethrough, it is necessary that either (a) an acoustic medium, typically a fluid, having suitable acoustic characteristics, be provided to transmit the ultrasonic waves; or (b) that the lens be pressed tightly against the flexible sealing medium. With the latter solution, the curved, generally cylindrical, shape of the lens deforms the sealing medium when the array is rotated, distorting the acoustic imaging beam. For the former solution, the acoustic medium is particularly necessary to fill the space between the focusing lens, which is typically curved, and the generally flat sealing film. However, the rotating curved lens causes churning of the acoustic medium and may also cause the outer covering film to distort, disturbing the acoustic waves passing through both the medium and the film. This is particularly true for a cylindrical lens which is preferable for focusing each element of the array, but which causes edge turbulence in the medium.
Further, the thin film which is used to seal the opening in the housing has little structural strength and is, therefore, subject to both distortion which may adversely affect acoustic imaging and to rupture. Therefore, it is generally desired to have a structural backing for this thin film layer. However, the acoustic properties of the backing for the thin film sealing element should not cause undesired distortion of the ultrasonic signals either transmitted or received by the transducer array. It is also desirable that a radio frequency interference (RFI) shield be provided between the body and the array to reduce noise in the array output.
A need, therefore, exists for an improved ultrasonic transducer probe which permits ultrasonic signals to be transmitted from a rotating transducer array through a stationary housing and seal, and in particular for an improved technique for accomplishing this objective in a transesophageal or other invasive probe.
Similar problems may also arise for noninvasive probes where a precise location is required for various imaging planes. In such application, the probe could be placed against the body and the array rotated to obtain different sector scans. Another nonrotational area where similar problems arise is in vascular probe imaging near the surface, or in other similar applications, where a vertical standoff of the probe is required to permit the beam to properly focus or to get adequate beam width for a sector scan. To avoid beam distortion, it is desirable that the offset area between the body and the probe not be filled with air.